1. Field of the Invention
The present invention relates to the on-line measurement of the moisture content of paper sheet at various stages of its manufacture as well as the on-line measurement of the basis weight of the paper.
2. Discussion of Background
The moisture content measurement of sheet paper in the prior art generally falls into the categories of microwave and infrared moisture gauges which can be used for real-time, on-line measurement. Each of the microwave and infrared moisture gauges used in the prior art have several deficiencies common to the type of structure used.
Poor spatial resolution and inadequate accuracy at moisture levels of less than 10% are serious disadvantages to the use of microwave devices. The second and more popular of the currently employed moisture content gauges uses infrared radiation which is usually produced by a filtered thermal source. These devices are more accurate than the microwave gauges because they use two wavelengths produced by alternating transmission filters in front of the source. One of these wavelengths is much more strongly absorbed in water than the other wavelength and from the use of these two wavelengths a fairly reliable water content measurement is possible. It is to be noted, however, that adequate results from using infrared gauges are often difficult to obtain because of the significant IR (infrared) scattering and poor penetration in heavier stock paper. It is also to be noted that the use of infrared gauges is confined to low moisture levels. Since the IR radiation is not confined to a well defined beam as laser radiation is, it is difficult, if not impossible, to steer the radiation for purposes of scanning a significant width of material as the sheet passes rapidly through the newsprint plant machines at up to 90 kilometer per hour. As a consequence, mechanical scanning of the whole device is required. An example of one type of infrared gauge structure is shown by the U.S. Patent to Brunton, U.S. Pat. No. 3,405,268 which illustrates the use of three different wavelengths of infrared radiation in the 1 to 10 micron range in order to measure moisture content and basis weight of paper as well as other dielectric sheet materials. This exemplary showing of an infrared gauge has all the drawbacks mentioned above including its use being confined to light stock paper and to paper which has low moisture levels. Likewise this gauge produces radiation which cannot be steered in order to provide adequate scanning of the width of the paper as it moves through the machines, and hence mechanical displacement of the whole device is required in order to produce the scan.
Neither the microwave nor the infrared type gauges are able to produce a beam in which the radiation is confined and which can be steered, as a laser beam for example, in order to provide fast, accurate, high spatial resolution, and wide range of moisture content measurements over a wide range of dry stock paper material.
There has been a recent attempt to provide for only the moisture measurement of a thin paper sheet using a laser as reflected by "Paper Sheet Moisture Measurements in the Far Infrared" Conference Digest, Eighth International Conference on Infrared and Millimeter Waves by Boulay et al, December, 1983. The theoretical discussions were confined to the use of a single wavelength metallic guided-wave CH.sub.3 OH laser with a hole coupled end mirror which is pumped with a CO.sub.2 laser whereby the output of the laser is directed at normal incidence onto a immobile paper sheet. This theoretical discussion provide only a measurement of a single wavelength with a quasi continuous wave laser output. This approach with respect to using the hole coupled end mirror arrangement on the output of the CH.sub.3 OH laser spreads the laser beam by diffraction and makes it extremely difficult to use for scanning across a width of material as is necessary in machine paper production operations.